1. Field of the Invention
This invention relates to a master cylinder applied to the hydraulic brake system of vehicles and, more particularly, to a master cylinder capable of delivering a large quantity of working fluid during the initial stage of the operating stroke thereof, while producing a high pressure in the latter stage of the working stroke thereof.
The master cylinder of such type includes a cylinder having a stepped bore and a stepped piston slidably fitted in the stepped bore of the cylinder jointly forming a first pressure chamber within the section of a large diameter of the bore and a second pressure chamber within the section of a small diameter of the bore. The first pressure chamber communicates with a resevoir through a check valve which allows flow of the working fluid from the reservoir to the first chamber when a negative pressure is effective in the first pressure chamber and through a relief valve which allows flow of the working fluid from the first pressure chamber into the reservoir when the pressure effective within the first chamber exceeds a predetermined pressure. The master cylinder is adapted to allow flow of the working fluid from the first pressure chamber into the second pressure chamber through the periphery of a cup seal attached to the piston section of a small diameter of the stepped piston due to the pressure difference between the pressures of the first and second pressure chamber.
2. Description of the Prior Art
It is well known in the master cylinder art that the pressure chamber needs to communicate with the reservoir to fill the pressure chamber with the fluid contained in the reservoir when the brake system is not actuated. In the case of a conventional master cylinder, the first pressure chamber communicates continuously with the reservoir by means of a second narrow passage separately formed in order to meet such need.
However, such a conventional construction of the master cylinder has the disadvantage in that the quantity of the fluid flowing from the first pressure chamber into the second pressure chamber is affected by the brake pedal operating speed, thus resulting in a variation of the stroke of the brake pedal. For example, when the brake pedal is pushed inwardly very slowly, most of the quantity of the fluid to be delivered from the first pressure chamber into the second pressure chamber flows into the reservoir through the narrow passage formed between the first pressure chamber and the reservoir so that an excessively long stroke of the brake pedal is necessary to raise the pressure in the second pressure chamber.